The present invention pertains generally to structural materials for fission and fusion-energy-generating systems and particularly to a niobium-base alloy resistant to irradiation-induced hardening and swelling in a high-temperature and high-neutron-flux environment.
The environment of nuclear-energy-generating systems is characterized by high temperatures and high-neutron fluxes throughout the system lifetime. These two characteristics create numerous problems for metals used as construction materials. At a high temperature, a high-neutron flux causes displacement reactions which create point defects of vacancies and interstitials. The point defects often migrate to form line defects termed dislocations to form planar clusters called loops, or to form three-dimensional clusters called voids. It has been determined that dislocations, loops and voids cause a metal to swell and to harden. Hardening results in the metal losing ductility which diminishes the service life of reactor components made from these metals. Void formation and growth also produce dimensional changes in the metal which reduce the service life of the components.
Commercial, nuclear-energy-generating systems utilize either water or helium gas as a coolant. The water-cooled systems operate under the relatively mild conditions of a temperature from 200.degree. to 300.degree. C. and a neutron flux such that the neutron fluence is less than approximately 10.sup.22 neutrons/cm.sup.2 during the service life of the system. At these conditions, the currently used materials, e.g. austenitic steels and nickel-base alloys do not undergo an excessive amount of hardening and swelling in view of their cost. In other words, the degradation in the properties of the alloys does not exceed the economy of using these metals.
The systems cooled by helium gas can operate at temperatures from 300.degree. to 600.degree. C. and have a neutron fluence in excess of 10.sup.23 neutrons/cm.sup.2 during their service life. Unfortunately, the presently used alloys, e.g. austenitic steel and nickel-base alloys perform poorly at the upper limits. Since these alloys are seriously degraded at a neutron fluence of approximately 10.sup.22 neutrons/cm.sup.2 if they were used at operating temperatures from 500.degree. to 600.degree. C., the system can not be operated at their capacity.
Alternatives to austenitic steel or nickel base alloys include ceramics and the niobium-1 weight percent zirconium alloy. Ceramics, such as silicon carbide or nitride have excellent temperature resistance, but have extremely poor ductility. Further, these ceramics degrade upon exposure to neutron radiation. Although the zirconium-base alloy perform slightly better at the higher temperatures of 500.degree. to 600.degree. C. than the presently-used alloys, the additional cost of the alloy makes its use impractical.
Nuclear-energy-generating systems using a liquid metal are in the developmental stage. The operating temperatures and neutron fluxes are similar to the gas-cooled systems. Consequently, these systems are similarly hindered by the limitations of presently-used alloys in their construction.
Considerable experimental work is being conducted on magnetic fusion energy-generating systems. These systems operate at temperatures from 400.degree. to 800.degree. C. and require alloys to be serviceable at neutron fluences from about 10.sup.24 to 10.sup.26 neutrons/cm.sup.2. It is evident that the presently used alloys would be completely inadequate for such systems.